The object of the present invention is a method and an apparatus according to the introductory parts of the independent claims presented below for separating solid or drop-like particles from air flows to be purified.
Then the invention relates particularly to a method and an apparatus where particles are separated from an air flow or a corresponding gas flow in a separator chamber or similar, by electrically charging the particles to be separated, e.g. by conventional high voltage techniques or ionisation, and by separating these electrically charged particles with the aid of a grounded collector surface or similar. Typically the separation occurs in an elongated cylindrical chamber, where at one end there is formed an air flow inlet and at the other end there is formed an outlet for the purified air flow which is separated from the particles. The electrode, ion generating means or similar which charges the particles is typically arranged in the central part of the separator chamber, so that it extends generally from the air flow inlet end to the purified air flow outlet end.
In conventional separators based on the electrical charge of the particles, as well as in electrical filters, the aim has been to obtain as low air flow velocities and as little turbulence as possible, so that the dust particles can be separated from the air flow in a smooth and controlled manner, and so that they are prevented from re-mixing with the air flow. The air flow is typically directed to the separator chamber via an inlet opening at the centre at one end of the chamber, and then the air flow is allowed to pass smoothly in the axial direction through the separator chamber, whereby the air flow and the separated particles uniformly fill the whole chamber, both at the centre and in the peripheral regions. Such conventional purification equipment must be made relatively large in order to provide the desired purification effect.
In conventional separators particles are separated from the air to be purified on all separator surfaces being in the way of the flow, both on the electrodes, on the structures supporting the electrodes and on the isolators arranged between the electrodes and the grounded collector surfaces. The particles accumulated on the surfaces will easily cause blockages which interfere with the free air flow through the separator chamber. On the other hand, piles of dry particles accumulated on the surfaces may from time to time begin to move again, as dust clouds, and form dusty air mixtures which impair the desired purification result. Further, layers accumulated particularly on the isolators may cause a short circuit between the electrodes and the grounded collector surfaces, which interferes with the operation of the isolator. Thus, due to the particles accumulated on the surfaces, the manageability and control of conventional purification equipment has often been difficult.
In addition to the small dust particles the air flow to be purified which flows through the separator chamber can sometimes also contain large particles or bodies which, when they hit the electrodes and stick to the electrode support structures, can both damage the structures and cause blockage.
Previously it has been proposed to use water spraying to wet the dust particles and bind them to each other in order to provide a better separation. However, the water will evaporate relatively rapidly from the moistened dust fog, and thus this wetting has not provided the desired result.
The American patent publication U.S. Pat. No. 4,388,089 presents an improved separator into which the gas to be purified is supplied in the tangential direction from above, so that the gas is made to pass through the separator in a generally spiral path around a smooth and simple wire electrode on the central axis of the separator whereby the centrifugal force automatically directs a part of the particles toward the grounded walls of the separator. There is further arranged a water film on the separator walls which flows from the top downwards, and with this film the aim is to lead in a controlled manner the particles which have separated on the separator walls away from the separator, and another aim is to prevent the particles from being re-mixed with the gas flow.
The capacity of a separator provided with a simple wire electrode is relatively low. In the above described case the use of other more efficient electrodes is limited i.a. by the fact that particles are more easily accumulated on their surfaces, which causes above mentioned blockage problems, or that the electrical field in their central part can be so low that the dust containing air flowing downwards in the central part will pass through without being purified. Particularly when the gas flow to be purified is large it may be difficult to prevent the particles in the central part of the separator, and at a distance from the water film to follow the gas downwards into the purified gas discharge at the bottom of the separator. Already due to the gravity force the particles tend to propagate directly downwards.
The objective of the present invention is thus to provide a new method and apparatus which are better than the previously known, in order to separate particles from an air flow.
The objective is to provide a method and an apparatus, where particularly the above mentioned drawbacks are minimised. Then an objective is particularly to provide a method and apparatus which minimise the accumulation of particles separated from the air flow on the electrodes, on the structures supporting the electrodes and on the isolators, and which minimise the problems caused by the accumulation.
An objective is to provide a method and an apparatus which enable the use of effective electrodes for charging the particles in the separator.
In order to achieve the above mentioned objectives the method and the apparatus according to the present invention are characterised in what is presented below in the characterising clauses of the independent claims.
The invention is particularly well suited for separating solid and drop-like particles from air flows to be purified in connection with the manufacturing of paper, paperboard, pulp or similar.
In a typical solution according to the invention the air flow to be purified is arranged to travel along a spiral path through an elongated vertical separator chamber, from an inlet at its bottom end to an outlet at its upper end, whereby the separator chamber acts as a separator where both electrical forces and the centrifugal force act on the particles and direct the particles toward a grounded collector surface at the periphery of the chamber. The collector surface is typically formed by the wall of the separator chamber, and the wall is grounded.
A typical separator according to the invention comprises a vertical cylindrical separator chamber, into whose central part an elongated electrode, ionising means or similar means with an open structure is arranged axially, which means can electrically charge the particles to be separated. The most different such electrodes can come into question where the electrode's horizontal open section has a large area, preferably over 90%. The electrode can be formed for instance of vertical wire electrodes or similar which are placed adjacent each other at short mutual distances in the form of a circle, of a net formed by wire electrodes which is bent into the shape of a cylinder, or of a smooth narrow cylinder, which is provided with sharp points or pins which initiate the discharge.
According to the invention the air flow to be purified is arranged to travel around an electrode or similar along a spiral path directed upward from the bottom. Then particles are separated from the air flow when they are exposed to the action of the centrifugal force and/or when they are electrically charged and directed toward the walls of the separator chamber. In a preferred solution according to the invention the air flow is arranged to pass spirally upwards along the walls of the separator chamber, so that the particles or bodies in the air flow will pass over the electrodes, the structures supporting the electrodes and the isolators, mainly without touching them.
Further, a continuous water film or water skin is preferably arranged in the separator chamber so that it flows along the walls of the chamber and immediately binds the dust particles or other particles when they contact the water. The turbulence occurring in the spirally upwards flowing air flow furthers the particles coming into contact with the water film, whereby even a minor water film flowing along the walls is enough to bind the dust. The particles brought into contact with the water film are wetted and bound to the water film, and cannot again mix with the air flow and form harmful dust-air mixtures. The water film flows downwards and moves the impurities stuck to it into the lower part of the separator chamber and discharges them from the separator.
The rotary motion of the air flow in the separator chamber results in that the main part of that air flow which is brought into contact with the electrodes, with the structures supporting the electrodes and with the isolators, is relatively clean and will not cause substantial accumulation of dirt on the structures. With the solution according to the invention it is possible to ensure that the electrodes and other surfaces at the centre of the separator chamber will be kept clean by creating conditions in the central part of the separator chamber around the electrode surfaces or similar for a downward directed flow of clean gas which washes the electrodes. The electrodes or electrode surfaces are designed with such a structure that they do no substantially hinder the gas flow at the region of the electrodes, but enable the clean gas to flow in the central part of the separator from the top of the separator chamber downwards. The clean gas can flow almost straight downwards, or in a small spiral downwards. In the solution according to the invention we have surprisingly realised to utilise the fact that in a separator where the supplied gas is brought to flow spirally upwards there is formed at the centre of the spiral flow a downward directed flow of clean gas, as far as there is arranged an open space for the clean gas flow in the central part of the separator. In the solution according to the invention the open space is arranged in the central part of the separator by arranging at the central part of the separator such electrodes which have a cross-section which is as open as possible, which in other words causes as little hindrance to the flow as possible. In the case according to the invention the open area of the electrode area is preferably over 90%.
Thus in the solution according to the invention it is ensured that the electrodes are kept clean by arranging an air flow in the separator chamber so that at the centre of the spirally upward flowing air flow between the electrode or similar and the wall of the separator chamber we allow the generation of a smaller downward directed flow of clean air. The downward flowing clean air washes the surfaces of the electrodes arranged in the central part of the separator chamber and removes any particles accumulated on them, and directs these particles downward to the particle discharge and/or directs them outward toward the spiral air flow where the particles will be subject to a new separating process. The flow of clean air is less than 30% of the air flow to be purified. Often a substantially smaller clean air flow is sufficient to keep the electrode surfaces clean.
In order to ensure the open area the electrodes can preferably be made of wire electrodes by arranging them at short mutual distances, or by bending them into electrode assemblies with a desired form, for instance a net form, however, leaving space between the electrode surfaces for the gas flow, or by forming sharp points or pins of the electrodes which are fastened to the support structure at the centre of the separator.
The spiral flow of the air to be purified is provided for instance by supplying the air flow in a tangential direction into the separator chamber from an inlet at the bottom part of the chamber. On the other hand the air flow can also be supplied axially from an inlet opening or channel arranged at the centre of the chamber when one or more blades or similar are arranged in the opening or channel, whereby these blades bring the air flow to pass along a spiral path in the chamber around the imaginary axis of the chamber.
The air flow is supplied into the separator from its lower part so that the air flow rotates against the internal surface of the cylinder. The air travels in a screw motion from the inlet toward the outlet. The rotational motion ensures a stable flow and ensures the same retention time in the separator for the whole air flow. Any large bodies in the air flow will hit and stick to the wall of the cylinder acting as a collector surface immediately when they come into the separator, without causing any harm.
Due to the rotational motion of the air flow the distance which the air flow travels in the separator chamber is many-fold, typically 3 to 6 times, compared to the length of the separator chamber, or compared to the distance which the air flow travels in conventional separators operating with an axial flow.